It seems to me that it would be useful if MouseEvent class had a property
that would enable listners to learn about coordinates of the event within
the element's own coordinate system. Currently, there are only properties
clientX, clientY that show coordinates relative to document and screenX,
screenY that show coordinates with respect to screen.
However, it would be very useful to have something linke offsetX, offsetY
that would show mouse coordinates relative to the origin of the element
(e.g. a DIV element) within which the event occured. Netscape already
offers this in form of Event.layerX, Event.layerY and Internet Explorer
offers this in form of Event.offsetX, Event.offsetY. However, none of those
properties are standard part of DOM.